Cloud Clipboard by Jett Higgins

Cloud Clipboard lets you quickly send text and images across all your devices.
How it works:

Create a basic account (just username and password - no email or phone required - Sign in once and stay logged in)
Put your text or images into the text boxes.
"Send to Phone" saves it to your cloud clipboard
Log in on your other device and click paste

Key features:
Works as a Firefox extension and as a Progressive Web App at cloudclipboard.app
On mobile, visit the website and click "Add to Home Screen" for app-like experience
Your most recent upload is always available
Secure account storage

Note: Only your most recent upload is stored at a time. Uploading a new one replaces the last.